Apparatus for vacuum casting molten metal



Dec. 27, 1960 s. E. PELLISSIER, JR 2,965,937

APPARATUS FOR VACUUM CASTING MOLTEN METAL Filed Feb. 21, 1957 Da ll lNVENTOR GEORGE E. PELLISS/ER, Jr..

his Attorney United States Patent APPARATUS FOR VACUUM CASTING MOLTEN METAL George E. Pellissier, Jr., Murrysville, Pa., assignor to }Jnited States Steel Corporation, a corporation of New ersey Filed Feb. 21, 1957, Ser. No. 641,588

4 Claims. (CI. 22-73) This invention relates to the vacuum casting of molten metals and more particularly to the elimination of explosion hazards in the vacuum casting of molten metals.

Considerable amounts of molten metals have been cast in vacuum to substantially eliminate gaseous impurities such as hydrogen from the molten metal. If steel, for instance, is cast into a mold contained in a chamber having a pressure less than .5 mm. the hydrogen is reduced to less than .00015%. Due to leakage of air, evolution of hydrogen from the metal being cast, moisture in the refractories used in the chamber, etc., there always exists a possibility of generating an explosive mixture within the vacuum tank during teeming of molten metal thereinto despite taking care to prevent such occurrence. In starting up such equipment, elaborate precautions are required when starting up vacuum casting equipment to eliminate all leaks. Combustible gases such as hydrogen, carbon monoxide, and simple hydrocarbons, are given off by the molten metal during casting. In the absence of oxygen such gases present no combustible problems, and there is no danger of fire or explosion. However, this is an ideal situation and 2,965,937 Patented Dec. 27 1960 the hazards of developing .a combustible mixture in vacuum casting tanks.

It is a further object to eliminate the hazard of develop ing a combustible mixture during casting in an efiicient and automatic manner.

The foregoing and further objects will be apparent from the following specification when read in conjunction with the attached drawing, wherein:

The figure is a vertical section of vacuum casting apparatus embodying the principles of my invention.

Referring more particularly to the drawing, the numeral 2 designates a vacuum casting chamber enclosed by a base member 4 and a removable cover member 6 with a suitable seal 8 therebetween. The cover is sufficiently strong to withstand inner pressures of the order of .5 mm. or less and support a removable basket ladle 10 on a seal 12. Flow of metal from theladle is controlled by a conventional stopper-rod assembly 14. If desired a fusible diaphragm 16 may be disposed below the nozzle 18 of the ladle so that the chamber 2 can be evacuated before molten metal is poured into the ladle 10. A suitable mold may be supported inthe tank or chamber 2 on' a base 20.

An exhaust duct 22 is connected to the base 4 of the tank 2 and leads to a suitable evacuating pump (not shown). The exhaust duct may contain gas sampling ports 24, an air inlet valve 26 and a main shut-oif valve 28. a

In accordance with the teachings of my invention, a jacket 30 is disposed around the cover 6 and is provided with suitable seals 32 and 34. This provides an enin practice cannot be achieved because of decomposition of oxides and water vapor, but mainly because of the virtual impossibility of preventing leakage of air into the chamber, and, providing a pumping system with sufiiciently high capacity to rapidly evacuate the leaking air from the chamber. One means that has been suggested to prevent the formation of a combustible mixture in the casting chamber is to attempt to balance the inleakagc rate of air, pumping speed, the rate of evolution of combustible gas from the molten metal and the time between initial evacuation and start of casting to lessen the danger of developing a combustible mixture. This is acomplished by reducing the inleakage rate of air to a very low level, by initially purging the chamber with an inert gas such as helium, by controlling the rate of pouring and by adhering to a prescribed time interval between final pump-down after purging and start of casting.

In practice, this balance is very diflicult to achieve and maintain because of the difficulty of predicting or anticipating with any degree of certainty the composition and amount of combustible gases given ofi by the molten steel, at various time intervals during the casting, but mainly because of the uncertainty as to what the inleakage rate of air will be under the casting conditions, when the vessel heats up and the vacuum seals may be loosened. Also, any delay in casting after final pump-down would either entail some danger of explosion, because of development of a combustible mixture, or involve a time-consuming repetition of purging with inert gas and pumping it. Furthermore, any inleakage of air into the chamber increases the amount of oxygen, nitrogen, and moisture in the casting atmosphere, and so tends to defeat the main purpose of vacuum casting.

It is accordingly an object of this invention to eliminate closed, sealed, annular space 36 of at least several inches in thickness around the chamber 2. The jacket 30 is provided with a connection 38 to a source of inert gas such as helium under suitable pressure.

The source of inert gas is also connected to a vertical tube 40 in the chamber 2 so that the chamber can be purged if desired. The tube has a series of graduated orifices 42 which increase in size towards the top thereof. This pipe 40 is diametrically opposite the opening of the exhaust duct 22 so that fresh purging gas is not sucked into the exhaust line immediately upon entering the vacuum chamber. The orifice openings 42 are graduated in size, being smallest at the lower end and becoming progressively larger toward the upper end to promote more uniform distribution of the purging gas in a short time interval within the vacuum chamber. A thermocouple vacuum gage 44 may also be provided in the chamber 2.

If desired, the outer jacket 30 may be welded to the inner cover so as to render it substantially leakproof to the atmosphere. This reduces the number of seals to those necessary for disassembling the inner vacuum chamber, and also facilitates the disassembling operation, since the outer jacket is an integral part of the inner vacuum chamber.

Thus it is seen that the novel arrangement of my invention provides a pressurized blanket or protective cover of an inert gas, such as for example, helium or argon, contained in a second outer chamber 36, which will insure that essentially no oxygen, carbon dioxide, or water vapor in the air surrounding the vacuum casting vessel will enter the vacuum chamber 2, through the inevitable leaks that exist in the various vacuum seals, during the vacuum casting operation. The pressure of the inert gas blanket is somewhat higher than the ambient atmospheric pressure, so that inert gas may leak both to the surrounding atmosphere and into the vacuum casting chamber, but air from the atmosphere cannot leak into this interposed chamber containing inert gas into the inner, vacuum casting chamber. Thus any gas that leaks 3 through the vacuum seals into the vacuum chamber must be the inert gas, and this small amount of inert gas at very low pressure is not deleterious to the vacuum casting process. In other words, because inleakage of gas ,to.

the vacuum chamber through seals is inevitable, I have substituted an inert and harmless gas, such as helium or argon, for air, which is reactive and deleterious to the process, as the inleaking gas. This means that the control of the gas composition in the vacuum chamber during casting is not nearly so difficult and critical, and this facilitates the operation and markedly reduces the explosion or flammability hazard. The inert gas is supplied from a high-pressure cylinder or flask, through a reducing valve to control the pressure of the inert gas in the outer chamber at some prescribed lower level above atmospheric pressure. As a small amount of gas leaks from the outer chamber, it is continually and automatically replaced by additional new inert gas from the high-pressure cylinder.

While I have shown and described several specific em bodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised Within the scope of my invention, as defined in the appended claims.

I claim:

1. Apparatus for casting molten metal in a vacuum including a shell forming a chamber adapted to be evacuated to a very low pressure, means in said chamber for receiving molten metal through sealing means, the combination therewith of a second shell enclosing said firstnamed shell providing a space therebetween, means for supplying gas under pressure into the space between said shells to provide a seal and prevent inleakage of air into said chamber and a vertically extending pipe in said chamber connected to a source of inert gas under pressure, said pipe having a plurality of orifices therein of gradually increasing size from the bottom towards the top.

2. Apparatus for casting molten metal in a vacuum including a shell forming a chamber adapted to be evacuated to a very low pressure, means in said chamber for receiving molten metal through sealing means, the combination therewith of a second shell enclosing said first-named shell providing a space therebetween, means for supplying gas under pressure into the space between said shells to provide a seal and prevent inleakage of air into said chamber and a vertically extending pipe in said chamber connected to a source of inert gas under pres sure.

3. Apparatus for casting molten metal in a vacuum including a base, member, a removable annular shell adapted to be disposed on said base forming a chamber adapted to be evacuated, an annular seal between said annular shell and said base, means in said chamber for receiving molten metal and means for delivering molten metal into said chamber through sealing means, the combination therewith of a second chamber enclosing said first mentioned chamber and said annular seal, said second chamber containing an inert gas under pressure to prevent inleakage of air into said first mentioned chamber.

4. Apparatus as in claim 3 characterized by means for admitting gas under pressure into said first mentioned chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,339,192. Roberson Jan. 11, 1944 2,713,183 Winkler July 19, 1955 2,782,475 Wilhelm et al Feb. 26, 1956 2,784,961 Coupette et a1. Mar. 12, 1957 2,796,644 Kuhn June 25, 1957 2,810,635 Cooper Oct. 22, 1957 FOREIGN PATENTS 736,852 Great Britain Sept. 14, 1955 

